This invention relates to the production of superplastically formed metal alloy panel structures, and is more particularly directed to the production of superplastically formed and diffusion bonded metal panel or sandwich structures, and the procedure for producing such structures.
Superplasticity is the characteristic demonstrated by certain metals to develop unusually high tensile elongations with minimum necking when deformed within a limited temperature and strain rate range. This characteristic, peculiar to certain metal and metal alloys has been known in the art as applied to the production of complex shapes. It is further known that at these same superplastic forming temperatures the same materials can be diffusion bonded with the application of pressure at contacting surfaces.
Diffusion bonding refers to the solid-state, metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a time duration so as to effect intimate surface contact and cause comingling of atoms at the joint interface.
U.S. Pat. No. 4,304,821 to Hayase, et al discloses the structure and the process for making a metallic sandwich structure or box section panel in which metal worksheets, preferably made from titanium alloy, are joined in a preselected pattern by an intermittent or discontinuous weld. The joined sheets are sealed by a continuous weld to form an expandable envelope. Application of inert gas pressure to the envelope in a limiting fixture superplastically produces the sandwich structure as the expanded structure diffusion bonds to itself or other worksheets. Core configuration of the structure is determined by the weld pattern.
In U.S. application Ser. No. 709,695, filed Jun. 3, 1991, of R. G. Pettit and Ken K, Yasui, and assigned to the same assignee as the present invention, there is disclosed a stiffened lightweight panel structure comprising a first core panel or core element comprised of superplastic material and including a plurality of successive inwardly extending superplastically formed substantially similar first bulges or cells, and at least one other core panel or core element comprised of superplastic material adjacent the first core panel, and including a plurality of successive inwardly extending superplastically formed substantially similar second bulges or cells orthogonally disposed to and extending toward the first bulges, the second bulges formed around and contacting the first bulges, and the first and second bulges being diffusion bonded at the contacting areas. In this structure the bulges or cells of the respective core panels are of substantially the same size and geometric configuration, and the adjacent core panels and the bulges thereof are disposed normal to each other.
It is an object of the present invention to provide an improved panel structure or sandwich structure by superplastic forming and diffusion bonding.
Another object is the provision of a stiffened, lightweight panel structure with good resistance to bending, shear and buckling.
A further object is to provide improved stiffened panel structures having the aforementioned characteristics, formed from four to eight workpieces or sheets.
A still further object is the provision of improved superplastically formed panel structures wherein the size and geometry of the bulges or cells of the adjacent core panels of the panel structure can be varied and the orientation of the respective core panels can be varied, with respect to each other to increase or optimize structural efficiency.
Still another object is to provide a panel structure having passages for heat exchanger or laminar flow control applications.
Yet another object is the provision of superplastic forming and diffusion bonding procedure for producing the improved stiffened panel structures or sandwich panels of the invention.
Other objects and advantages of the invention will appear hereinafter.